142 MAN AND THE GLACIAL PERIOD. 



this age far-travelled stones are to be found, but they are 

 always accounted great rarities. 



" The Eed Crag consists of an irregular assemblage of 

 beaches and sand-banks of widely different ages, but their 

 sequence can be made out with ease by a study of the 

 fauna. In the oldest deposits, Mediterranean species are 

 very numerous, while the boreal forms are comparatively 

 rare ; but in successive later deposits the proportions are 

 very gradually reversed, and from the overlying Chilles- 

 ford series the Mediterranean species are practically ab- 

 sent. The physical indications run pari passu with the 

 paleontological, and in the newer beds of the Eed Crag 

 far-travelled stones are common. 



" In the Forest Bed series there is a marine band — the 

 Leda myalls bed — which contains an almost arctic assem- 

 blage of shells ; while at about the same horizon plant 

 remains have been found, including such high northern 

 species as Salix polaris and Betula nana. 



" The glacial deposits do not, in my opinion, contain 

 anywhere in England or Wales a genuine intrinsic fauna, 

 such shells as occur in the East Anglian glacial deposits 

 having been derived in part from a contemporary sea-bed, 

 and, for the rest, from the older formations, down perhaps 

 to the Coralline Crag. In the post-glacial deposits we 

 have hardly any trace of a survival of the boreal forms, 

 and I consider that the whole marine fauna of the North 

 Sea was entirely obliterated at the culmination of the 

 Glacial epoch, and that the repeopling in post-glacial 

 times proceeded mainly from the English Channel, into 

 which the northern forms never penetrated. 



" The Great Glacial Centres. 



" Where such complex interactions have to be described 

 as were produced by the conflicting glaciers of the British 

 Isles it is difficult to deal consecutively with the phe- 

 nomena of any one area, but with short digressions in ex- 



